I'd like to discuss hypothetical systems we could use the describe the location of Earth in the galaxy in a sci-fi universe.

Astronomers have perfectly good naming and address systems, for their purposes. In particular there are two tendencies I'd like to avoid.1) Enumerate, don't name: The IAU's "naming" convention for stars doesn't allow for any actual naming. Anyone who's had to read someone else's code can appreciate the practical difference between a well named variable and a variable with a meaningless identifier.2) Stuck on the surface of a sphere: Astronomy tends to be subjective in that it describes things in terms of light approaching Earth.

So, what would our address be with a good sci-fi system? The best I've come up with:

Local fluff and Local Bubble need better names. Apparently Sol isn't in a star cluster, so I'm guess cloud and bubble are the appropriate lower level structures.

7 Orion is the position of our bubble in the Orion Arm. The number being the number of degrees from the arm's start, potentially going above 360 for arms that loop around the galaxy. The arms start being the inner end.

Good question -- I remember thinking of a similar question for some sci-fi I was writing a while back.

I like the idea of defining with respect to the arm, but what if you're in a small cluster between major arms, or what if the arm disappears over a few billion years? Since we're in a barred spiral galaxy, I'd suggest we start by defining a north-south convention based on the axis of the bar surrounding Sagittarius A. We can let the axis of the bar serve as the demarcation between the northern half of the galaxy and the southern half of the galaxy, with Sol being on the north side.

Our galaxy has two major arms, the Scutum-Centaurus Arm and the Perseus Arm. Combined with our earlier demarcation, this allows us to define four major regions of our galaxy: Perseus North, Perseus South, Centaurus North, and Centaurus South. On this basis, the Orion Spur is located almost exactly along the border between the Perseus North and Centaurus North regions.

We'd probably want to use degrees to define a compass position for any given location, thus locating it along the length of the named arm. Using the direction of galactic rotation as our "clockwise", we start with 0 degrees North and sweep 180 degrees across (crossing Sol at approximately 135 degrees), then start at 0 degrees South and sweep 180 degrees back around to our starting position. So "45 Centaurus North" identifies the point where 45 degrees North crosses the center of the Centaurus Arm, while "15 Perseus South" identifies the point where 15 degrees South crosses the center of the Perseus Arm. By this measure, Sol is almost exactly equidistant between 135 Centaurus North and 135 Perseus North.

We can then locate ourselves more precisely by moving in toward the center of the galaxy from the arm or out away from the center of the galaxy from the arm. Something like "Inner Sectors A-Z" could describe moving toward the galactic center while "Outer Sectors A-Z" could describe moving away from the galactic center. For example, "45 Perseus South, Outer Sector M" locates you outside the Perseus South Arm at a cardinal direction of 45 degrees South, about halfway between the arm and the edge of the galaxy. "90 Centaurus North, Inner Sector B" would be just inside the Centaurus North Arm, perpendicular to the bar of the galaxy.

By this measure, the region around Sol could be described either as "135 Perseus North, Inner Sector Z" or "135 Centaurus North, Outer Sector Z" depending on whether you want to measure inward from the Perseus North Arm or outward from the Centaurus North Arm.

EDIT: On second thought, there's no need to introduce the whole North/South nonsense. Just specify 360 degrees with the central galactic bar defining 0 and 180, then you can name any location by reference to the cardinal angle and the nearest galactic arm.

For a coordinate system, I'd define up/down as with /against gravitational potential (outward/inward), east/west as with/against gravitomagnetic procession (spinward/anti-spinward), north/south as directions along the access of rotation. This differs from the galactic coordinate system only in the choice of center and in being cylindrical rather then spherical.

what if the arm disappears over a few billion years?

Well I'd say language should change every few thousand years. If a civilization last long enough to for the arms to change it could have an enumeration (coordinate) system that persists underneath, and generate meaningful names as the stars move.

135 Perseus North, Inner Sector Z

One of the things I dislike is that coordinates tell you nothing about what's there. Cradle of civilization? Foreign power? Maw of malevolent outer god? Big wad of emptiness?

My old suggestion is far from perfect in this regard. "Local" fluff and bubble are far from great, but they at least say "humanity started here". The 7 in "7 Orion" is just a coordinate, but I'd estimate there's room for 25 thousand local bubble size structures in Orion, much more in the larger arms. I couldn't really think of anything better than a coordinate to bridge that change in magnitude.

what if you're in a small cluster between major arms

hmmm.. well the solution seems to me to define the regions between arms as arm-scale regions. Arm scale regions being arms, unarms (spaces between arms), the core, and rim segments (spaces outside outer arms but still in the galaxy). I'll just entirely table the issues posed by the galactic spheroidal halo.

As for naming the unarms, I'd suggest generally using the name of the higher arm, but with lots of exceptions. Similar to how seas are named "sea of <nearby land area>", but it's not always clear which land area it "belongs to", and sometimes it has enough personality to get it's own name.

If you look at this image, I would call the region above Sagittarius but below Turner 5 and Cygnus X the "Orion unarm". The region west of Tuner 5 "Perseus unarm west". The region between below Perseus and above Orion "Perseus unarm east", this region would continue to include the space above Sagittarius but below Perseus. The regions below Cygnus and above Perseus would be called "Cygnus unarm west" and "Cygnus unarm east", with Orion dividing the two.

The thing about recursion problems is that they tend to contain other recursion problems.

For a coordinate system, I'd define up/down as with /against gravitational potential (outward/inward), east/west as with/against gravitomagnetic procession (spinward/anti-spinward), north/south as directions along the access of rotation. This differs from the galactic coordinate system only in the choice of center and in being cylindrical rather then spherical.

Interesting.

I think there are two conflicting factors in play, though. From a galactic standpoint, the thickness of the main disk is almost negligible in comparison to the galaxy's diameter. So cylindrical coordinates make a lot of sense; after all, we ARE dealing with what is essentially a very flat cylinder.

On the other hand, the thickness of the galaxy is not at all negligible from a local perspective. 1000 lightyears is a pretty big distance. When we look up at the night sky, most of what we're seeing along the galactic plane is within the Orion Spur, though the galactic bulge is of course quite visible too.

135 Perseus North, Inner Sector Z

One of the things I dislike is that coordinates tell you nothing about what's there. Cradle of civilization? Foreign power? Maw of malevolent outer god? Big wad of emptiness?

Perhaps what we need is a two-level system -- the first level which places you in a general region of the galaxy, and the second level which identifies your location by reference to the nearest subgalactic superstructure.

Let's use this image as a galactic-level reference:

Spoiler:

And here's one with a few superstructure labels:

Spoiler:

You'll notice that the New Outer Arm can be constructed as an extension of the Outer Arm, which itself can be constructed as an extension of the Norma Arm. Similarly, the Carina Arm is the opposite end of the Sagittarius Arm, which is the region the Orion Spur passes through. So we have four major superstructures we can identify...the Centaurus Arm, the Sagittarius-Carina Arm, the Norma-Outer Arm, and the Perseus Arm:

I agree that it's hard to think of anything other than coordinates for bridging the really huge changes in magnitude that we're dealing with here. We should probably do our best to minimize the use of bridging coordinates. But how?

If we define an ordinal direction and use degrees, I think there's some promise:

The "bridging coordinates", then, would be of the form (heading)+(arm). For example, "270 Perseus" identifies the entire region at which heading 270 crosses the Perseus Arm, and "40 Sagittarius" identifies the entire region at which heading 40 crosses the Sagittarius Arm (which is roughly where we are).

Each of the arm-sections as I've defined them above are roughly 10,000 lightyears wide. Since the galactic disk is 1,000 lightyears thick, the top-level bridging coordinate defines an ellipse-shaped cross-section of the given arm, ten times wider than it is thick:

At our distance from the galactic center (roughly 26,000 ly), a single degree of arc defines a region about 370 lightyears wide on the inner end and 540 lightyears wide on the outer end, so the top-down view of the region defined by, say, "0 Perseus" would look like this:

Using just the single bridging coordinate, then, gives us a region of space usually around 10,000ly x 1,000 ly x 450 ly, for a total volume of roughly 130 million cubic parsecs. Within this region, we can define an absolute numeric coordinate system (probably angle+distance, though a cartesian approach is also possible), but regions would probably have local names based on nearby crossing structures within the arm.

So you could define any point by using sufficiently precise coordinates (using the angle+distance approach, Sol would be at something like 39.982° Sagittarius, 89.99960°920.021 parsecs), but typical usage would be to give the bridging coordinate of 40 Sagittarius followed by local identifiers: Inner Sector, Pleiades Cavity, Lupus Tunnel, Altarian-Centaurian Cloud, Sol. Inner Sector points you to the inside of the Sagittarius Arm, Pleiades Cavity directs you to the 300-lightyear-wide bubble centered around the Pleiades supernovae remnants, Lupus Tunnel points you to the region of the Pleiades Cavity bordering the nearby Scorpius Association Cavity, and Altarian-Centaurian Cloud points you to the gas cloud containing Altair, Centauri, and Sol (among other stars).

This is similar to Earth, where any point can be identified exactly using latitude and longitude, but it is more common to give an overall region, followed by progressively more precise regional identifiers.

sevenperforce wrote: For example, "270 Perseus" identifies the entire region at which heading 270 crosses the Perseus Arm, and "40 Sagittarius" identifies the entire region at which heading 40 crosses the Sagittarius Arm (which is roughly where we are).

What would 170 Perseus be? The heading goes through the arm twice.

Pleiades Cavity, Lupus Tunnel, Altarian-Centaurian Cloud

I approve.

So we have four major superstructures we can identify...the Centaurus Arm, the Sagittarius-Carina Arm, the Norma-Outer Arm, and the Perseus Arm

That seems a reasonable way to do the highest level structure, but it seems to me we could get some mileage out of the minor arm structure.

I'll call the four arm structures you named "Whorls".

"Spurs" will be the next level structure down. This will include minor arms that have a different angle then the whorls (like Orion). The major arms tend to wax and wane along their length; that's why you have the situation where arm C is an extension of arm B, which is an extension of arm A. A, B, and C would be different "spurs" in the same whorl.

it's hard to think of anything other than coordinates for bridging the really huge changes in magnitude that we're dealing with here. We should probably do our best to minimize the use of bridging coordinates

One thing I didn't include was a reference to the Gourd Belt. Which would be intermediate between the Altarian-Centaurian Cloud and the Orion spur. I didn't include it because I just know what's in the wikipedia article, and that leaves me super unsure about what it's an instance of.

If "belt" isn't a coherent categorization, we'd probably need some kind of coordinate to bridge that gap. My original recommendation was relative to the spur, because that's the gap we'd need to bridge.

It someone wants to be technical, they'd use a galactic cylindrical system with the appropriate number of significant figures (seven, if you want to be clear on the difference between alpha and proxima centauri).

The thing about recursion problems is that they tend to contain other recursion problems.

sevenperforce wrote: For example, "270 Perseus" identifies the entire region at which heading 270 crosses the Perseus Arm, and "40 Sagittarius" identifies the entire region at which heading 40 crosses the Sagittarius Arm (which is roughly where we are).

What would 170 Perseus be? The heading goes through the arm twice.

The outermost part could be 530 Perseus, as it's gone around the galaxy once.

gmalivuk wrote:

King Author wrote:If space (rather, distance) is an illusion, it'd be possible for one meta-me to experience both body's sensory inputs.

Yes. And if wishes were horses, wishing wells would fill up very quickly with drowned horses.

Quizatzhaderac wrote:What would 170 Perseus be? The heading goes through the arm twice.

In cases where a particular heading crosses an arm more than once, there would probably be a simple identifier like "170 Perseus A" vs "170 Perseus B" or "90 Norma I" vs "90 Norma II".

Pleiades Cavity, Lupus Tunnel, Altarian-Centaurian Cloud

I approve.

So we have four major superstructures we can identify...the Centaurus Arm, the Sagittarius-Carina Arm, the Norma-Outer Arm, and the Perseus Arm

That seems a reasonable way to do the highest level structure, but it seems to me we could get some mileage out of the minor arm structure.

Well, simply using a 360-heading in combination with one of those four arm names will "drill down" to a region of space approximately 10,000 ly x 1000 ly x 500 ly. That's a lot of space, but it's far enough down that spur structure is no longer a dominating factor; local topography will be dominated by voids and cavities left over from supernovae.

By analogy, one could identify points on Earth using the following geological hierarchy:

This would work fairly well. However, if I gave you coordinates for Earth that gave you longitude (on a 200-mile scale) and continent, you probably wouldn't need to bother with identifying the subcontinental plate. That's not because the region wouldn't quite possibly cross multiple plates, but because you could skip straight to local fault line features.

In the same way, having a 360-heading and the name of a "whorl" arm can locate you precisely enough that your next step down will be supernovae cavity structures rather than bothering with a particular spur.

It someone wants to be technical, they'd use a galactic cylindrical system with the appropriate number of significant figures (seven, if you want to be clear on the difference between alpha and proxima centauri).

The reason I suggested using the "heading + arm" approach was because it can serve as the first step of the more technical system without any need for a completely different set of coordinates.

I think there's value in being able to choose between "40 Sagittarius, such-and-such Cavity, such-and-such Cloud" and "40.049 Sagittarius, 71.99387 degrees 422.011 parsecs", particularly because the first identifier matches more closely. Plus, if you're giving "directions" within your own region, you can drop the whorl name entirely and people will understand, even if you're using technical coordinates.

Given the cross-section of a region defined by a top-level identifier like "40.049 Sagittarius" (which I depicted in one of the images above), I'm not sure whether cartesian or cylindrical coordinates is a better fit. Any thoughts on this?

For the record, you need about three decimal digits on your compass heading to pinpoint a 4-lightyear-wide band, which is typically good enough for picking out individual star systems (the Hill sphere of a sun-like star is about 1 lightyear across, so gravitationally unbound systems will typically be at least 1-2 lightyears apart).

sevenperforce wrote:Given the cross-section of a region defined by a top-level identifier like "40.049 Sagittarius" (which I depicted in one of the images above), I'm not sure whether cartesian or cylindrical coordinates is a better fit. Any thoughts on this?

I presume for the Cartesian coordinates you'd use the height in galactic plane, galactic radius and whatever happens to be perpendicular to those two at heading 40.049?

I'd say at that scale getting right angles out of your coordinate system is more important than following the galactic curve.

For the record, you need about three decimal digits on your compass heading to pinpoint a 4-lightyear-wide band, which is typically good enough for picking out individual star systems (the Hill sphere of a sun-like star is about 1 lightyear across, so gravitationally unbound systems will typically be at least 1-2 lightyears apart).

Right, three decimal points after a degree makes six significant figures, if you convert to turns the number of sigfigs becomes really clear. The seventh sigfig is just to make sure the area of ambiguity is smaller than common hill spheres.

I think there's value in being able to choose between "40 Sagittarius, such-and-such Cavity, such-and-such Cloud" and "40.049 Sagittarius, 71.99387 degrees 422.011 parsecs", particularly because the first identifier matches more closely. Plus, if you're giving "directions" within your own region, you can drop the whorl name entirely and people will understand, even if you're using technical coordinates.

sevenperforce wrote:Given the cross-section of a region defined by a top-level identifier like "40.049 Sagittarius" (which I depicted in one of the images above), I'm not sure whether cartesian or cylindrical coordinates is a better fit. Any thoughts on this?

I presume for the Cartesian coordinates you'd use the height in galactic plane, galactic radius and whatever happens to be perpendicular to those two at heading 40.049?

Nah, the origin would be the point designated by "40.049 Sagittarius", that is, the point on the galactic plane at which heading 40.049 crosses the idealized centerline of the Sagittarius whorl. So you're using something like the following:

To the left (away from the galactic core) would be designated as the positive x-direction; up (relative to Earth's North Pole) would be designated as the positive y-direction. Then you could use typical cartesian coordinates to identify any point. Obviously, you could also use cylindrical coordinates here by using an angle (setting 0 as north and moving counterclockwise) and a distance from the origin.

The advantage of this over using a "galactic radius" approach is that you decrease the size of your x-coordinate to something much more manageable, and you allow for cylindrical coordinates to function half-decently. One function of a good coordinate system is the ability to roughly place a location in your mind just by looking at the numbers. But if you use the galactic core as your origin, the x-coordinate will be useless for visualizing the location.

If the coordinate is something like 110 Perseus 48,770.4 ly, -203.1 ly (galactic core origin) then you really don't know where it is, other than the general region of the Perseus arm. However, if the coordinate is 110 Perseus -44.4 ly, -203.1 ly then you immediately know "Oh, it's 44 lightyears inside that segment of the Perseus arm, 203 lightyears under the galactic plane." You can immediately visualize a pretty specific area, whereas you can't if your origin is the galactic core.

If you use cylindrical coordinates, the difference is even more stark. Coordinate locations in inner arms will have a completely different angular range (from the galactic core) than coordinate locations in outer arms, so you would need way more significant figures in outer arms. But if your origin is the arm you're placed in, then coordinates become more consistent from arm to arm. The coordinate 21°395 ly means a star system above and slightly outside the nearest galactic arm, regardless of which arm it is and whether it's closer to the start or end of that arm.

sevenperforce wrote:Nah, the origin would be the point designated by "40.049 Sagittarius"

Okay, that's kind of what I was picturing, I just phrased my question very poorly.

110 Perseus -44.4 ly, -203.1 ly

That describes an volume about 500 ly by 0.1 ly by .1 ly .

The extra precision could be added in two ways: Adding decimals to the galactic coordinate or adding a third local coordinate. The third local coordinate would enable the local coordinates to be fully Cartesian.

The thing about recursion problems is that they tend to contain other recursion problems.

As an example of why arm-centered origins are better than core-centered origins...

This is a hypothetical cross-section of the galaxy, taken across the 285-105 line. If you used cartesian coordinates with a galactic core, you would end up with VERY large x-coordinates and only comparatively very tiny y-coordinates. It's even worse if you tried to use spherical coordinates, as you can see:

The vectors pointing to any position past the first galactic arm are all basically flat, so they would have very little meaning at first glance. In contrast, there's a big different if you use arm-origin cylindrical coordinates:

At least now the angles will be consistent from arm to arm and not nearly so flat.

Quizatzhaderac wrote:

sevenperforce wrote:

110 Perseus -44.4 ly, -203.1 ly

That describes an volume about 500 ly by 0.1 ly by .1 ly .

The extra precision could be added in two ways: Adding decimals to the galactic coordinate or adding a third local coordinate. The third local coordinate would enable the local coordinates to be fully Cartesian.

Yes, adding precision to the initial coordinate is probably the route to go. Using a third local coordinate wouldn't do much good because there is no physical structure within an arm to correspond to each of the 360 compass points.

Even without the additional precision, this still comes closer to the overall goal of identifying by reference to galactic structures rather than depending on numbers alone. It allows you to look at an overall map of the galaxy and place a pin in the approximately correct place because you're using the shape of the arms as your local reference rather than just guessing position based on distance from the center.

It's similar, by analogy, to some of the differences between map projections (e.g., what's more important, getting the distances right or getting the shapes right?). For our purposes, it's probably more important to make sure you can locate yourself relative to nearby structures than anything else.

I was thinking more about this and I can't help thinking that being able to "trim" unnecessary information from an address/location is one of the most important aspects of coming up with a good coordinate system.

For example, suppose you're visiting me and you ask where the nearest public library is. I'm not going to say "Northern and Western Hemispheres, North America, United States, Eastern Time Zone, Georgia, Atlanta, 30339, 4290 Paces Ferry Road Southeast." In fact, I can omit everything but the last phrase, and you won't have any trouble finding the place, because you automatically assume anything not specified is simply the same as wherever you're starting from.

For a galactic coordinate system, having meaningful but consistent high-level regions would be a big advantage. To that end, I wanted to revisit the notion of quadrants, because that (in combination with the arm designations we came up with earlier) can break up our galaxy into more manageable regions.

If we define cardinal directions as follows:

NSEW only smaller.png (241.98 KiB) Viewed 8605 times

...then this allows us to define four galactic quadrants.

Labeled Quadrants.png (231.44 KiB) Viewed 8605 times

What does this accomplish? Not much, yet. But earlier, we agreed on four "whorl" arms, as labeled here:

If we combine the arms with the quadrants, we end up with something potentially more useful:

What we've done is separate the galaxy into sixteen sectors, all of which are roughly 1000 lightyears thick and 10,000 lightyears wide. The eight inner and outer sectors are the smallest at about 35 kly lightyears long, while the other eight sectors range from 40 kly to 55 kly in length. The volume of the sectors, then, ranges from 10 kpc to 16 kpc.

With this arrangement, it becomes possible to identify nearly any location in a sector to within a few lightyears using only 7 digits, if we take advantage of positive and negative values. Instead of using angles or set units, we can just split each sector up into evenly-sized sections. Let's use the North Perseus sector as an example.

The first three digits can represent the distance from the nearest galactic pole within that sector, going around the curve of the arm. The clockwise direction is positive. Coordinate 000 would be on the North galactic pole; coordinate 500 would be halfway between the North galactic pole and the northeast edge of the quadrant; coordinate -750 would be 3/4 of the way from the North galactic pole toward the northwest edge of the quadrant, and so forth:

Since this sector is approximately 40 kly in length, going from -999 all the way to +999 breaks this sector into about 2000 "slices" each only 2 lightyears wide.

That takes care of the tangential location.

The next two digits can represent the distance above or below the galactic plane. Since all arms are roughly 1000 lightyears wide, using only two digits (-99 to +99) will break this up into sections approximately 5 lightyears thick:

vertical.png (6.7 KiB) Viewed 8605 times

In the above example, the vertical axis is stretched to show detail.

Finally, we want to orient ourselves radially within each sector. Each sector is consistently 10,000 lightyears deep, so in order to split this into sufficiently precise sections we should probably use letters rather than numbers as follows:

-ZZ, -ZY, -ZX...-AC, -AB, -AA, +AA, +AB, +AC...+ZX, +ZY, +ZZ

262 gives us 676 values, so from -ZZ to +ZZ is 1,352 values, splitting our 10,000-lightyear-deep sector into sections each about 7 lightyears deep:

depth.png (8.81 KiB) Viewed 8605 times

A full coordinate, then, would look something like West Sagittarius -663.09.GY or South Centaurus 280-71-JX or North Perseus 90.45.XY (obviously, vertical position not shown):

Each of these coordinates narrows you down to a volume of roughly 2 cubic parsecs. Most importantly, if you're naming a location within the same sector, you don't have to name the quadrant or arm; you can just say "130-17AR" and people will know where you mean.

Then when we start colonizing other galaxies, we'd need a coordinate system to specify which of the hundred billion galaxies...but cosmologists have mapped the observable sky, and there it's not at all homogeneous, there are plenty of clumps, clusters and empty spaces.

sevenperforce wrote:A full coordinate, then, would look something like West Sagittarius -663.09.GY or South Centaurus 280-71-JX or North Perseus 90.45.XY (obviously, vertical position not shown):

Each of these coordinates narrows you down to a volume of roughly 2 cubic parsecs. Most importantly, if you're naming a location within the same sector, you don't have to name the quadrant or arm; you can just say "130-17AR" and people will know where you mean.

Honestly, I prefer the earlier system of

sevenperforce wrote:So you could define any point by using sufficiently precise coordinates (using the angle+distance approach, Sol would be at something like 39.982° Sagittarius, 89.99960°920.021 parsecs), but typical usage would be to give the bridging coordinate of 40 Sagittarius followed by local identifiers: Inner Sector, Pleiades Cavity, Lupus Tunnel, Altarian-Centaurian Cloud, Sol. Inner Sector points you to the inside of the Sagittarius Arm, Pleiades Cavity directs you to the 300-lightyear-wide bubble centered around the Pleiades supernovae remnants, Lupus Tunnel points you to the region of the Pleiades Cavity bordering the nearby Scorpius Association Cavity, and Altarian-Centaurian Cloud points you to the gas cloud containing Altair, Centauri, and Sol (among other stars).

I feel that it sufficiently provides for the sort of localization you suggested while maintaining human readability. The fact is that absolute identifiers don't need to be localizable anyway, a human will almost never use an absolute identifier, rather, they'll use local galactic features as you suggested in your original system, and no matter how localizable you make a name, alphanumeric coordinates will never be human readable.